Add support for use as pre-commit hook

[proinsias]

• Adds configuration files needed for image_optim to be used as pre-commit.com hook.
• Also adds GitHub Action to regularly run pre-commit autoupdate and create a pull request to update hook versions if needed.

[toy]

Thanks for contribution.
Unfortunately looks like the action failed.
Could you also please describe what is the action supposed to do? Check if version is up to date with what pre-commit knows as the latest version?

[proinsias]

@toy – thanks for the review. I believe I've addressed your comments. I realized I didn't need the GitHub Action as you're not using pre-commit for this repo itself.

[toy]

Thanks for finishing the PR :)
Travis is failing only because failing check on up to date rubies.
Seemed to work when installing from your fork and specifying commit sha for rev, do I understand right that it would be better to create a release (tag) for it to work nicely?

[proinsias]

Thanks! And yes a release tag would be great.

[toy]

Created v0.29.0 tag